Most tractors, whatever their application, typically comprise a non-steerable driven axle, and a steerable axle, the wheels of which may or may not be driven.
In a typical arrangement the steerable axle is pivotally mounted to the tractor chassis to pivot about a generally horizontal axis. The front axle is thereby permitted to oscillate with respect to the chassis, so that the wheel may stay in contact with uneven ground.
A number of physical constraints are imposed on the range of movement of the steerable wheels. Physical constraints include tyre size, track setting, clearance between chassis and the end of the axle etc.
The maximum turning angle provides the smallest turning diameter for any particular tractor set up. In order to increase maneuverability of the tractor, designers have sought to increase the turning angle.
One solution to the problem of increasing turning axle has become known generally as compound steering. In a compound steering mechanism a decreased turning radius is achieved by moving the steering axle relative to the chassis at the same time as turning the steering wheels. By moving the axle relative to the chassis, the turning angle may be increased in two ways. First, in many compound steering mechanisms the axle moves through an arc so that instead of extending at 90 degrees to the longitudinal axis of the tractor, and direction of travel it extends of between 80–90 degrees to the direction of travel, depending on the direction of the steerable wheels. Second, by moving the axle relative to the chassis, the distance between an inner steerable wheel and the chassis is increased thereby permitting the maximum angle of the inner steerable wheel and the front axle to be increased.
An example of a compound steering mechanism is found in U.S. Pat. No. 5,046,577. In the steering mechanism described in U.S. Pat. No. 5,046,577, when the steerable wheels are turned relative to the steering axle simultaneously, the steering axle pivots relative to the chassis. A connecting link interconnects the pivot mechanism which pivotally connects the steering axle to the chassis, with the steering mechanism such that manipulation of the steering mechanism effects a turning of the wheels and a pivotal movement of the steering axle in the same direction the wheels are being turned. The pivot mechanism allows for pivotal movement of the steering axle about a longitudinally extending horizontal axis in addition to the pivotal movement of the steering axle about a generally vertical axis relative to the chassis. A lost motion linkage interconnecting the pivot mechanism and the steering axle accommodates differences in arcuate movements due to the pivot mechanism and the steering axle pivotally moving about longitudinally spaced pivot centres.
The compound steering mechanism of U.S. Pat. No. 5,046,577 is complex requiring a number of moving parts. Such a mechanism is both costly to manufacture and maintain.
Another compound steering arrangement is described in U.S. Pat. No. 5,312,123. In the compound steering arrangement described in this patent a steering axle is mounted on one surface which carries a vertical pivot and a ball joint which is connected to the tractor rearwardly of the steering axle. The front of the subframe is attached to the tractor by a yoke comprising a carrier member pivotally attached to the tractor and a pair of dog bone links, each link being attached to the carrier member and the axle. The assembly provides for pivotal movement of the steering axle relative to the chassis by a four bar linkage so that the tractor chassis maintains a substantially uniform elevation relative to the steering axle throughout the range of movement of the steerable axle. This compound steering mechanism requires many moving component parts and as such is expensive to manufacture and maintain.
Another compound steering arrangement is described in European Patent 0400033. The arrangement comprises a tubular drag link which at one end is rigidly attached to the steering axle, and at its other end is attached to the tractor chassis by means of a ball joint, the ball joint being rearward of the steering axle. The maximum turning angle is further enhanced by positioning the steerable axle such that during a turn the front of the tractor chassis. The practice of positioning the front axle such that during a turn the front of the outside turning wheel moves in front of the front end of the tractor chassis is not unique to compound steering arrangement. Many agricultural tractors have this feature.
The front of the steerable axle is also connected to the tractor chassis by either a pair of pivotable springs or control arm.
The arrangement described in European Patent 0400033 requires a costly tubular pole type drag link and modification to the tractor chassis to accommodate said drag link. Furthermore, the length of the drag link places design limitations on the provision of drive to the front axle. Also, the length of the drag link may improve stability problems.
It would therefore be desirable to provide an improved compound steering mechanism.